The present invention relates to a magnetic bubble memory device of a cassette type, which serves as a portable data recorder.
In general, in a non-volatile large-capacity memory device, high reliability is necessary for storing and holding a large quantity of information. Although a magnetic tape device or magnetic disk device is non-volatile, it has a fatal disadvantage in that it involves movable parts. Therefore, these memory devices, even though they are non-volatile large-capacity memory devices, do not have high reliability.
In recent years, a magnetic bubble memory device has been widely used as a non-volatile large-capacity memory device having a high reliability. In particular, since a cassette-type magnetic bubble memory device (hereinafter, referred to as a cassette) is small in size and weight, the cassette can serve as a portable data recorder and be used for entering data in a computer. The cassette is freely mounted and demounted in a cassette body device having a driving circuit, a read-out circuit, a bubble control circuit and the like.
Since the cassette is a kind of magnetic memory, the bubble information stored in the cassette may be destroyed when subjected to an external magnetic noise field. For this purpose, a magnetic bubble memory chip is protected by a shield case, so that the bubble information is not destroyed even when subjected to an external magnetic noise field whose strength is 4000 to 20000 A/m (amperes per meter). In addition, a space between the shield case and the cassette case is always provided, so as to create a magnetic shield effect. However, just a shield case and a space is not enough, and accordingly, it is necessary to provide additional means for protecting against strong external noise magnetic fields to which a cassette may be subjected.